1. Field of the Invention
The present invention relates generally to scanning and copying documents and, more particularly, to a method and apparatus for reproducing sepia-tone original documents.
2. Description of Related Art
Optical scanners have long been used to capture existing images so that they may be copied, transferred, stored, and even digitally manipulated. A scanner captures the image by illuminating the image with a light source and sensing reflected light. The presence or absence of reflected light at a particular point (picture element or “pixel”) determines the image characteristics at that point. For a black and white image, the absence of reflected light indicates a black point and presence indicates a white point. For gray scale images, the intensity of the reflected light is measured to determine the intensity of the image at that point. In the case of color, separate light sources or filters are used to detect the presence or absence of each color component.
Document reproduction using commercially available copiers, scanners, and digital senders is progressing toward affordable, full-color, faithful duplication. It is known in the art to use primary color (red (R), green (G), blue (B)) scanning technology to render acquired data as a “faithful” reproduction, that is, producing a copy of the original exactly as it appears, whether or not there are areas of the document which have defects or artifacts.
Aged historical documents and photographs are often sepia tone. Certain modern “black and white” photographic films, specifically those that are amenable to convenient, automated, “one-hour,” color photoprocessing and color photopaper printing, generally render the original black-and-white negatives as sepia tone prints. The use of an RGB scanner for sepia tone originals, particularly where the original is damaged, does not render a desirable reproduction. If the image has aging artifacts, these defects are faithfully copied. For example, when a sepia tone photograph ages, the original “black” areas (full shadow) start to turn blue. The blue areas can be much lighter than the intended black, greatly reducing image quality. Using a color copier, any defects are also faithfully reproduced; namely the faded area appears as a noticeable blue area. If a black-and-white scanning-copying is performed, the reproduction is no longer a sepia print. Moreover, defects such as faded regions now appear too light.
Colorimetry has long been recognized as a complex science. Essentially, as defined in 1931 by the Commission Internationale L'Eclairage (CIE), three primary colors (X, Y, Z) can be combined to define all light sensations we experience with our eyes; that is, the color matching properties of an ideal trichromatic observer defined by specifying three independent functions of wavelength that are identified with the ideal observer's color matching functions form an international standard for specifying color. In general, it has been found possible and convenient to represent color stimuli vectors by a three-dimensional spatial construct, called a tristimulus space or a color space. The fundamentals of such three-dimensional constructs are discussed in the literature, such as Principles of Color Technology, by Billmeyer and Saltzman, published by John Wiley & Sons, Inc., NY, copyright 1981 (2d. ed.) and Color Science: Concepts and Methods, Quantitative Data and Formulae, by Wyszecki and Stiles, published by John Wiley & Sons, Inc., copyright 1982 (2d ed.), see e.g., pages 119-130.
A variety of trichromatic model systems provide alternatives for both the hardware and software system designers—e.g., the red, green, blue (RGB) model commonly used in computer video displays; the cyan, magenta, yellow (and black) (CMY(K)) model used extensively in color hard copy apparatus; the hue, saturation, value (HSV) model; the hue, lightness, saturation (HLS) model; the luminance, red-yellow scale, green-blue scale (L*a*b*) model; the YIQ model used in commercial color television broadcasting; and others. Such works as Fundamentals of Interactive Computer Graphics, by Foley and Van Dam, Addison-Wesley Publishing Company, see e.g., pages 606-621, describe a variety of tri-variable color models.
Color input and output devices—such a scanners, cathode ray tube (CRT) video monitors, and printers—present color images in a device-dependent fashion. Color model system data point values (voltage levels or other input signal functions, referred to hereinafter as data triplets or color coordinates) are digitally stored in a frame buffer. Color transformation (also referred to in the art as color correction and cross-rendering) between model systems in digital data processing presents many problems to the original equipment manufacturer. The transformation of data from one device to another device is difficult because the color matching relationship between those systems are generally non-linear. For most applications, a crucial problem is the maintaining of color integrity between an original image from an input device (such as a color scanner, CRT monitor, digital camera, computer software/firmware generation, and the like) and a translated copy at an output device (such as a CRT monitor, color laser printer, color ink-jet printer, and the like); i.e., presenting a “faithful” reproduction. A transformation from one color space to another requires complex, non-linear computations in multiple dimensions. Since such is not describable mathematically, it is known to use very large look-up tables to approximate a transformation between trichromatic model systems to correlate the wide spectrum of color hues that exist. A lookup table of input versus output data can be generated for any set of devices. There are a variety of methods for building a device-dependent look-up table for a particular device. The 1975 U.S. Pat. No. 3,893,166 to Pugsley provides an example. Other, more modern, conversion techniques are described in Dillinger et al., U.S. Pat. No. 5,870,077 for a METHOD FOR TRISTIMULUS COLOR DATA NON-LINER STORAGE, RETRIEVAL, AND INTERPOLATION; U.S. Pat. No. 5,748,176 by Gondek for a MULTI-VARIABLE COLORIMETRIC DATA ACCESS BY ITERATIVE INTERPOLATION AND SUBDIVISION; and U.S. Pat. No. 5,732,151 by Moon et al. for a COMPUTERIZED MEMORY MAPPING METHOD FOR TRANSFORMING COLOR DATA (each assigned to the common assignee herein and incorporated by reference hereby).
There is a need for methods and apparatus for sepia tone reproduction, providing image quality equal to the original document when new, correcting for image artifacts and defects caused by the aging process.